Microbial removal of the C-25 methyl group from the extended 17α,21β(H)-hopanes yields a complete series of 25-norhopanes in some subsurface petroleum reservoirs. C-25 demethylation favors low molecular-weight hopanes, while higher homologs are more resistant. Remarkably, conversion of each extend hopane to its corresponding 25-norhopane occurs without significant additional products as indicated by a match between reconstructed C31-C35 hopane distributions for heavily biodegraded oil (sum of hopane parent and 25-norhopane product for each homolog) and those for genetically related, nonbiodegraded oil. C-25 demethylation favors 22S vs. 22R epimers of the C31 and C32 hopanes, while the opposite applies to the C34 and C35 hopanes because molecular shape, dimension, and volume varies with stereochemistry at C-22. The most stable C31-C35 hopane 22S and 22R epimers show "scorpion-" and "rail-shaped" conformations respectively, controlled by differing 21-22-29-31 and 17-21-22-30 torsion angles. The 22S epimers of the extended hopanes favor the scorpion conformation, which folds the side chain back toward C-25, possibly hindering enzymatic attack. Two mysteries remain; Which microbes perform C-25 demethylation and why does it occur in some reservoirs but not in adjacent reservoirs under similar subsurface conditions?

Biodegradation of extended hopanes to 25-norhopanes in petroleum reservoirs: Insights from molecular mechanics

255th National Meeting (2018)

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